The infection of macaques by simian immunodeficiency virus (SIV) results in many of the characteristics of HIV infection and progression to AIDS in humans, including loss of CD4+ T cells and susceptibility to opportunistic infections. Our studies have used this SIV macaque model to focus on various vaccine approaches. 1. Role of cell-mediated immunity in AIDS vaccines. Many of the AIDS vaccine studies to date have focused on humoral immunity with an emphasis on the development of high- titered neutralizing antibodies. A possible role for cell-mediated immunity (CMI) in protection against HIV infection is suggested by the findings that a significant number of potentially HIV- exposed individuals from different risk groups fail to produce antibodies to HIV and show no signs of infection, but exhibit a strong HIV-specific cellular immunity. To test this hypothesis in the SIV-macaque model, we inoculated macaques with graded doses of SIV and measured the ability of macaque lymphocytes to proliferate in response to a panel of SIV envelope peptides. We have shown that subinfectious doses of SIV that do not result in an apparent infection in these animals nevertheless lead to an SIV-specific lymphocyte response, and that this response appears to confer long-term protection against a subsequent infectious virus challenge. The inability to productively infect macaques previously exposed to a subinfectious dose of SIV suggests that a T cell-mediated response may confer long-term protection against infection. These data may explain the observation that some commercial sex workers in Nairobi, although exposed to HIV-infected partners, are not themselves infected. Lymphocytes obtained from some of these women exhibit an HIV-specific cellular immune response, implying that they have been exposed to the virus. Future studies are planned that will inoculate pig-tailed macaques, both intrarectally and intravenously, with a subinfectious dose of either live or killed SIV/Mne. This study will address the question of whether the virus needs to be live in order to protect against a subsequent infection. This hypothesis assumes that at subinfectious doses there is a reservoir of virus that may be replicating at a privileged site and generating CMI. According to this hypothesis, killed virus delivered at a subinfectious dose would not be protective. 2. DNA AIDS vaccines. Studies are underway, in the SIV macaque model, to test the effectiveness of DNA immunizations in protecting macaques from a subsequent infectious virus challenge. The zinc-finger structures found in retroviral nucleocapsid proteins are necessary for packaging the retroviral genomic RNA during budding and maturation of the virus particle. Pig- tailed macaques (Macaca nemestrina), which are unusually susceptible to SIV infection, were immunized intramuscularly with DNA containing the SIV genome which had been rendered non-infectious by removal of a region of the nucleocapsid gag protein. None of 5 macaques immunized intramuscularly with this nucleocapsid mutant DNA showed any signs of a productive SIV infection, indicating that the DNA construct was safe. Antibody detected by ELISA after DNA inoculations indicated that the proviral DNA had been expressed. These macaques, as well as controls that were immunized with plasmid DNA, have been challenged with SIV. All controls became infected and have exhibited CD4 cell depletion. The DNA immunized macaques have not shown CD4 depletion to date, and exhibited a non-detectable viral load of SIV RNA in plasma as well as a much reduced ability to isolate virus from circulating lymphocytes. The results of this preliminary study suggest that DNA vaccines may be an effective and safe route of immunization leading to protection from an AIDS virus challenge.